Network storage controllers are typically used to connect a host computer system with peripheral storage devices, such as disk drives or tape drives. The network storage controller acts as an interface between the host computer and the peripheral storage devices. In many applications, the network storage controller performs processing functions on the data transferred between the host computer and peripheral devices. One common application of such a system is a Redundant Array of Independant Disks (RAID). A RAID system stores data on multiple disk drives to protect the data against disk drive failure. If one disk drive fails, then the RAID system is generally able to reconstruct the data which was stored on the failed drive from the remaining drives in the array. A RAID system uses a network storage controller, which in many cases includes a RAID controller, an interface between the host computer and the array of disk drives.
Many applications require a storage system to have very high availability. This high availability is a key concern in many applications, such as financial institutions and airline reservations systems, because the users rely heavily on the data stored on the RAID system. In these type of applications, unavailability of data stored on the RAID system can result in significant loss of revenue and/or customer satisfaction. Employing a RAID system in such an application enhances availability of the stored data, since if a single disk drive fails, data may still be stored and retrieved from the system. In addition to the use of a RAID system, it is common to use redundant RAID controllers to further enhance the availability of a storage system. In such a situation, two or more controllers are used in a RAID system, with each controller having failover capability, where if one of the controllers fails the other remaining controller will assume operations for the failed controller. Such a platform enhances the availability of a RAID system, however, it can lead to several disadvantages, as will be discussed below.
FIG. 1 shows a block diagram representation of a common current-day dual controller configured RAID network storage controller 10, showing a fiber channel to fiber channel connection. That is, in this example, the host computer and the array of disk drives both communicate with the network storage bridge using fiber channel connections. While fiber channel is a common channel medium is such systems, it should be understood that other channels may also be used, such as, for example, Small Computer System Interface (SCSI) or Ethernet. The RAID system shown in FIG. 1 includes two host ports, host port-1 14 and host port-2 18 and two disk ports, disk port-1 22 and disk port-2 26. Each host port 14, 18 may be associated with different host computers, and each disk port 22, 26 may be associated with different disk arrays, as is common in RAID systems and is well known in the art. The network storage bridge 10 includes dual RAID controllers, controller-A 30, and controller-B 34. In a system employing zoning of controllers, controller-A 30 may be zoned to host port-1 14 and disk port-1 22, and controller-B 34 may be zoned to host port-2 18 and disk port-2 26.
As is understood in the art, systems which employ dual controllers with write back caching require data mirroring between controllers to maintain cache coherency. Each controller 30, 34, must have a copy of the data and status of the other controller in order to maintain redundancy between the controllers and thus maintain operation of the RAID system if one controller fails. Mirroring data between controllers can decrease the performance of a RAID system because transferring data between controllers uses processing resources of the controllers, as well as channel bandwidth, as will be discussed in more detail below.
The controllers 30, 34 are connected to a fiber channel backplane 38, which is connected to two IO modules, IO module-1 42, and IO module-2 46. Each controller 30, 34, includes a CPU subsystem 50, a memory 54 (e.g., double data rate), control logic 58, a dual port fiber channel connection with two host ports 62a, 62b and a dual port fiber channel connection with two disk ports 66a, 66b. The CPU subsystem 50 performs tasks required for storage of data onto an array of disks, including striping data, and initiating and executing read and write commands. The memory 54 is a nonvolatile storage area for data and other information. The control logic 58 performs several functions, such as interfacing with the CPU subsystem 50, memory 54, and the host ports 62a, 62b and the disk ports 66a, 66b. The control logic 58 may also have other functions, including a parity generation function, such as an exclusive OR (XOR) engine. The host ports 62a, 62b and disk ports 66a, 66b provide communications with the fiber channel backplane 38. The IO modules 42, 46 include link resiliency circuits (LRCs) 70, also known as port bypass circuits, which function to connect each host port 14, 18 and each disk port 22, 26 to each controller 30, 34. This allows both controllers 30, 34 to have access to both host ports 14, 18 and both disk ports 22, 26.
In order to provide full redundancy, each controller must have a connection to each host port 14, 18 and each disk port 22, 26. This way, if there is a failure of one of the controllers, the other controller can continue operations. As mentioned above, it is common for each host port 14, 18 to be associated with different host computers, and for each disk port 22, 26 to be associated with different disk arrays. In these cases, each controller 30, 34 is typically associated with one disk port and one host port, which helps to enhance the performance of a RAID system. However, in such a case, half of these ports are passive. For example, if controller-A 30 is associated with host port-1 14 and disk port-1 22, then controller-A 30 receives all communications from host port-1 14 and controls the disk array(s) on disk port-1 22. Likewise, controller-B 34 would be associated with host port-2 18 and disk port-2 26. These techniques are well known in the art and can increase performance of the RAID system as well as simplify control and communications of the two controllers 30, 34. In the example of FIG. 1, on controller-A 30 the host port connection 62a and disk port connection 66a are connected to host port-1 14 and disk port-1 22, respectively, through the LRCs 70 of IO module-1 42. Because controller-A 30 is associated with host port-1 14 and disk port-1 22, the host port connection 62a and disk port connection 66a actively communicate with host port-1 14 and disk port-1 22. The remaining host port connection 62b and disk port connection 66b are connected to host port-1 18 and disk port-2 26, respectively, through the LRCs 70 of IO module-2 46. These connections are typically passive connections, as controller-A 30 is not actively communicating with host port-2 18 and disk port-2 26, so long as controller-B 34 does not fail. Likewise, controller-B 34 would be associated with host port-2 18 and disk port-2 26. Thus, for controller-B 34, the host port connection 62b and disk port connection 66b would communicate with host port-2 18 and disk port-2 26 through LRCs 70 of IO module-2 46. The remaining host port connection 62a and disk port connection 66a would be connected to host port-1 14 and disk port-1 22 through LRCs 70 of IO module-1 42.
As mentioned above, in typical redundant controller operations with write back caching data is mirrored between controllers. When mirroring data between controller-A 30 and controller-B 34, it is common to transfer the mirrored data over the disk port connections. For example, controller-B 34 may receive data over host port-2 18 that is to be written to an array of drives over disk port-2. Controller-B 34 would receive this data and store it in memory 54. In order to maintain cache coherency, controller-B 34 must also communicate this data to controller-A 30, thus both controllers have the data, and if one fails the other is still able to write the data. In a traditional system, this transfer of data is accomplished over several steps. First, controller-B 34 sends the data over the disk port connection 66a which connects to the LRC 70 connected to disk port-1 22. The data would transfer to the associated hardware on disk port-1 22 and be transferred back to the LRC 70, where it would then be received at the disk port connection 66a on controller-A. Controller-A would then store the data in memory 54, providing a copy of the data that was originally sent to controller-B 34. Controller-B 34 would then perform the appropriate steps to write the data to the disk array. Once the data is written to the disk array, controller-B 34 then communicates this to controller-A 30 using the same communication path as described above, and controller-A 30 then removes the record of the data write. Likewise, if controller-A 30 receives data to be written to the array of disks on disk port-1 22, it sends the data to controller-B 34 using the same mirroring technique.
While this technique may use the remaining disk port on each controller, the second host port on each controller remains unused, thus passive, during normal operation of the system. The passive ports on each controller adds a significant amount of hardware to the controller, and can add significant cost to the network storage controller 10. Thus, it would be advantageous to provide a redundant network storage controller which maintains high availability while reducing cost and hardware associated with passive ports located on the controllers.
Additionally, mirroring data in such a system results in the mirrored data and storage data being sent over the same port for the controller that is receiving the mirrored data. Bandwidth to and from the disk array is consumed by the mirrored data, which can reduce the performance of the network storage bridge. Additionally, when mirroring data, processing resources within the controllers 30, 34 are consumed, because the controller sending the data has to put it into form to be transferred over the disk port, and the controller receiving the data must process the data received over the disk port. For example, in the fiber channel embodiment shown in FIG. 1, mirrored data is formatted pursuant to fiber channel protocol, which can require several interrupts and processing resources. Thus, it would be advantageous to have a network storage controller which consumes little or no channel bandwidth when mirroring data between controllers. It would also be advantageous to have a network storage controller which consumes less processing resources for mirroring data.
Furthermore, with the continual increasing of demand for data storage, RAID controllers often require upgrades with additional disk drives or faster bus interfaces. However, a RAID controller may not be configured to add additional bus interface capacity or may not support a new type of bus interface. Such controllers commonly have to be replaced when an upgrade is performed. This replacement of controllers can increase the cost of upgrading a RAID system. The replacement of an operational RAID controller represents a loss in value that may inhibit the decision to upgrade a RAID system. Thus, it would be advantageous to have a system which can support upgrades of capacity, as well as new interface types, with ease and reduced cost.
Accordingly, there is a need to develop an apparatus and method for use in a network storage controller which: (1) provides redundancy with reduced cost for passive components, (2) reduces the amount of mirrored data which is sent over the disk or host ports, (3) reduces the processing overhead involved with mirroring data, and (4) provides easily replaceable and upgradeable components.